Virus-induced diseases in agronomically important crops have cost farmers a great loss of income due to reduced yields. Traditionally, virus diseases have been controlled by breeding for host plant resistance or by controlling insects that transmit diseases. Chemical means of protection are not generally possible for most viruses, and where possible are not generally practical. It has been known for many years that viral symptoms can be reduced in virus-infected plants by prior inoculation with a mild strain of the same virus, a phenomena known as cross-protection, as described by Sequeira, L., Trends in Biotechnology, 2, 25 (1984). Cross-protection is considered successful if the disease symptoms of the superinfecting (the more virulent) virus can be delayed or suppressed. There are several disadvantages to applying this type of cross-protection to the field situation:
1) application of the mild strain virus to entire fields is usually not practical, PA1 2) the mild strain might undergo mutation to a more highly virulent strain, PA1 3) the protecting strain might interact synergistically with a non-related virus causing a severe pathogenic infection, PA1 4) a protecting virus in one crop may be a severe pathogen in another crop, and PA1 5) a protective strain may cause a significant loss of yield in itself.
One proposed solution to these disadvantages has been to introduce a single viral gene into the host plant genome to cross-protect, rather than infect with an intact virus. This single gene cross-protection strategy has already been proven successful using the coat protein gene from tobacco mosaic virus (TMV-CP). As reported by Abel, P. P., et. al., Science, 232, 738 (1986), transgenic tobacco plants, expressing TMV mRNA and coat protein (CP), demonstrated delayed or suppressed symptom development upon infection with TMV. TMV-CP transgenic tomato plants have been described by Nelson, R. S., et. al., Bio/Technology, 6, 403 (1988), to show evidence of protection from TMV as well as three strains of tomato mosaic virus (ToMV). Other approaches using DNA clones of viruses to engineer resistance include positive interference, as described by Golemboski et al. Proc. Natl. Acad. Sci. USA, 87, 6311 (1990) and Carr and Zaitlin, Mol. Pl. Microbe Inter., 4, 579 (1991); and antisense RNA, as described by Powell et al., Proc. Natl. Acad. Sci. USA, 86, 6949 (1989).
Numerous viruses exist for which resistance is desired. Maize chlorotic dwarf virus causes a somewhat variable mosaic or yellow streaking and occasional stunting in maize. Early infections can result in severe symptoms including premature death. The virus is spread by the blackfaced leafhopper (Graminella nigrifons). MCDV can overwinter in Johnsongrass (Sorghum halepense) and as a result has become a recurrent problem in areas where Johnsongrass is a common weed. Combined infections with maize dwarf mosaic virus can cause more severe symptoms although the syndrome is less well characterized than Corn Lethal Necrosis. Only limited success has been obtained to date in developing MCDV-resistant maize lines, due to the difficulties of selecting effidently for resistance to an obligately insect transmitted virus, as well as a lack of usable sources of resistance in agronomically useful maize lines. Thus, there is a continuing need for genes, plant transformation vectors, and transformed plant materials providing resistances to pathogenic viruses such as MCDV.
Unfortunately, while certain plant viruses, such as tobacco mosaic virus, have coat protein genes that are found on subgenomic RNA and are therefore relatively easy to identify and clone for use in engineered cross-protection, maize chlorotic dwarf virus belongs to a completely separate group, the only other (tentatively assigned) member of which is the spherical virus of the rice tungro disease (RTSV). In addition, MCDV has a number of unusual biological properties which make identification of an appropriate gene difficult. For example, all attempts to mechanically transmit MCDV have been unsuccessful. As another example, MCDV appears to be a phloem-restricted virus. MCDV also has three coat proteins, and it was not known whether expression of one protein would be sufficient to confer immunity or whether all three would need to be expressed. Nor was it known which protein would be the appropriate one to express if only one could be expressed. Further, the genome of MCDV has an unusual genome organization to provide for the expression of multiple coat proteins.